In the past, the technique of FMIP has been known as a technique to provide effective means for Internet application requiring real time to minimize packet loss, which cannot be prevented by ordinary layer-3 handover using MIP technique (e.g. see the Non-Patent Document as given below.) Description will be given below on FMIP by referring FIG. 1, FIG. 41 and FIG. 42.
The radio communication system shown in FIG. 1 includes: IP network (communication network) 15 such as Internet, a plurality of subnets (also called subnetworks) 20 and 30 connected to IP network, a mobile terminal (MN: Mobile Node) 10, which can be connected to one of the plurality of subnets 20 and 30, and CN (Correspondent Node), which is correspondent of MN10. In FIG. 1, two subnets 20 and 30 are shown as the plurality of subnets 20 and 30.
The subnet 20 comprises an access router (PAR) 21 for executing the routing to an IP packet (packet data), and a plurality of access points (APs) 22 and 23 for forming inherent radio coverage areas (communicatable areas) 22 and 23. Each of these APs 22 and 23 is connected to PAR21, and PAR21 is connected to the IP network 15. In FIG. 1, two APs 22 and 23 are shown as the plurality of APs 22 and 23. Also, the subnet 30 comprises the same aspect of connection as the subnet 20 as described above by an access router (NAR) 31 and a plurality of APs 32 and 33.
Here, assumption is made on a case where handover is executed from the subnet 20 to the subnet 30 when MN10 moves in a radio coverage area 34 formed by AP32 via an overlap area 26 from a radio coverage area 25 formed by AP23. Hereinafter, the access router, which is present at a superior position to AP23 and to which MN10 is connected before the handover, is called PAR (Previous Access Router) 21, and the access router, which is present at a superior position of AP32 and to which MN10 is connected after the handover, is called NAR (New Access Router) 31.
PAR21, i.e. a component element of the subnet 20, and NAR31, i.e. a component element of the subnet 30, can communicate with each other via the IP network 15. That is, the subnet 20 and the subnet 30 are connected via the IP network 15.
Next, description will be given on operation in FMIP referring to FIG. 1. In FMIP, there are two operation modes, depending on whether FBAck (Fast Binding Acknowledgement) message is received or not on a link (link before handover), to which MN10 is connected before the handover. This depends on whether MN10 transmits FBU (Fast Binding Update) message or not on a link before the handover. Hereinafter, an operation mode in case MN10 transmits FBU message on a link before the handover is called a first operation mode, and an operation mode in case MN10 does not transmit FBU message on a link before the handover is called a second operation mode.
First, description will be given below on the first operation mode, in which MN10 transmits FBU message on a link before the handover. FIG. 41 is a sequence chart to show general features of the first operation mode in FMIP when MN10 transmits FBU message on a link before the handover.
For instance, when MN10 starts to move from an area of PAR21 (radio coverage area 25 of AP23) to an area of NAR31 (radio coverage area 34 of AP32), the movement is detected by a layer 2, and the handover in the layer 3 as starting point is initiated. This initiation of the handover is determined through comparison of the intensity of the received electric field from AP23 with that of the electric field from AP32 in the overlap area 26.
When information including AP-ID (identification information of each AP) of AP32, which is the destination of movement, is notified from the layer 2, MN10 transmits an RtSolPr (Router Solicitation for Proxy) message including AP-ID of AP32 to PAR21 currently connected (Step S801). Upon receipt of the RtSolPr message, PAR21 retrieves an access router existing in the neighborhood according to AP-ID of AP32 as notified from MN10 and acquires the information of NAR31, or it acquires the information of NAR31 from the information already retrieved (information retained by PAR21).
Then, PAR21 transmits a PrRtAdv (Proxy Router Advertisement) message including the information of NAR31 (e.g. link layer address of NAR31, or network prefix of the subnet 30, to which NAR31 belongs) to MN10 as a response to the RtSolPr message (Step S803).
Upon receipt of this PrRtAdv message, MN10 generates NCoA (New Care-of Address), which is an address adaptable at the subnet 30 by using the network prefix of the subnet 30 included in the PrRtAdv message and the link layer address of MN10 itself, and FBU message including NCoA is transmitted to PAR21 (Step S805).
Upon receipt of the FBU message, PAR21 transmits an HI (Handover Initiate) message including NCoA to NAR31 in order to confirm whether the NCoA generated at MN10 is an address usable at the subnet 30 or not (Step S807). When the HI message is received, NAR31 verifies whether the NCoA included in the HI message is effective or not. In case NCoA is effective, HAck (Handover Acknowledge) message to specify the status showing the result is transmitted to PAR21 (Step S809). When the HAck message is received, PAR21 transmits FBAck message to notify the result to MN10 and NAR31 (Steps S811 and S813). Then, the packet sent to MN10 is forwarded to NAR31 (Step S815). When the packet to MN10 has been forwarded from PAR21, NAR31 performs buffering of the packet.
Then, MN10 initiates actual moving to the subnet 30 and executes the handover from AP23 to AP32, for instance (Step S817). Immediately after connection switchover to NAR31, an FNA (Fast Neighbor Advertisement) message to request the notification of connection to NAR31 and the transmission of the packet processed by buffering is transmitted to NAR31 (Step S819). Upon receipt of FNA message, NAR31 transmits the packet processed by buffering to MN10 (Step S821).
Next, description will be given on the second operation mode, in which MN10 does not transmit the FBU message on a link before the handover and transmits FNA (message including FBU) on a link after the handover. FIG. 42 is a sequence chart to show general features of the second operation mode in FMIP when MN10 in the conventional technique transmits FNA [FBU] message on a link after the handover in the conventional technique (prior art).
Similarly to the first operation mode shown in FIG. 41, MN10 transmits RtSolPr message (Step S901) and receives PrRtAdv message (Step S903). Then, without transmitting the FBU message in the first operation mode shown in FIG. 41 (Step S805 in FIG. 41), actual moving to the subnet 30 is initiated. For instance, L2 handover is executed from AP23 to AP32 (Step S905).
Then, immediately after the connection switchover to NAR31, MN10 transmits the FNA message including FBU message in it (this message is described as FNA [FBU]) to NAR31 (Step S907). NAR31 verifies the effectiveness of NCoA included in the FNA message (Step S909). In case NCoA is effective, the FBU message is transmitted to PAR21 (Step S911). In case NCoA is not effective, NAAck (Neighbor Advertisement Acknowledgement) message to notify that NCoA cannot be used is sent to MN10.
As a response to this FBU message, PAR21 transmits FBAck message to NAR31 (Step S913) and forwards the packet addressed to MN10 to NAR31 (Step S915). NAR31 receives FBAck message from PAR21 and forwards the packet addressed to MN10 as received from PAR21 to MN10 (Step S917).
In the Non-Patent Document 1 as given below, it is suggested that IPv6 address of NAR31 is set as the destination address of the FNA message when the FNA message is sent to NAR31. In view of the fact that the setting of NCoA is given as the address of transmission source of the FNA message, there is no disclosure on the procedure, by which it can be estimated that IPv6 address of NAR31 is a global IP address (there is no disclosure on the procedure, by which NAR31 acquires global IPv6 address) or it is a multi-cast address in the subnet 30.
[Non-Patent Document 1] Rajeev Koodli; “Fast Handovers for Mobile IPv6”, draft-ietf-mobileip-fast-mipv6-08, October 2003.
[Non-Patent Document 2] T. Narten, E. Nordmark and W. Simpson: “Neighbor Discovery for IP Version 6 (IPv6)”, RFC2461, December 1998.
However, according to the technique disclosed in the Non-Patent Document 1, MN10 can quickly receive the packet immediately after the handover by performing the acquisition and the registration (Binding Update) of NCoA before or immediately after the handover, while due consideration is not given to the quick transmission of the packet by MN10 after the handover.
FIG. 43 shows a routing table to explain a first example of the problem in the conventional FMIP technique. To transmit the packet to outside from inside of the subnets 20 and 30, MN10 refers to the routing table. FIG. 43 shows the routing table before MN10 carried out the handover. When the packet is transmitted to outside (e.g. CN40) under the condition that MN10 before the handover is connected with the subnet 30, the entry relating to a default router (a router set in such manner that the packet can be sent to outside from within the subnet 20) is hit from the transmission destination of the packet, and then, the entry where the link local address of PAR21 specified to the gateway of the entry is set to the destination is hit. As a result, MAC address of PAR21 specified to the gateway of the entry is set as the destination address of Ethernet (registered trademark) of the packet. FIG. 43 shows the routing table where the default router of the subnet 20 is the same as PAR21.
On the other hand, in order that MN10 transmits the packet from within the subnet 30 after the handover, the link local address of NAR31 or the link local address of the default router 37 of the subnet 30 must be set up as a default router in the routing table of MN10. However, despite of the fact that MN10 can acquire new CoA (NCoA) by FMIP before the handover, the default router in the subnet 20 before the handover is still set up as the default router of MN10 after the handover in the routing table. In this respect, the packet to be sent to outside from the subnet 30 is abandoned without being sent from the subnet 30. Even when MN10 attempts to update the routing table immediately after the handover, it is not possible to update the routing table because NAR31 of the subnet 30, which is a new destination of connection or the link local address of the default router 37 is not known.
On the other hand, in the Non-Patent Document 2 as given above, it is disclosed that MN10 can acquire the external transmission address as described in the RA message (the link local address of NAR31 as given above or the link local address of the default router 37 of the subnet 30) by receiving an RA (Router Advertisement) message from NAR31 or the default router 37. Also, in the RA message, information to identify the default router 37 is also described. However, the RA message is the message to be sent out periodically from each router, and it is substantially difficult that MN10 transmits the packet to outside (e.g. CN40) until MN10 passively receives RA message and acquires the external transmission address. Similarly, besides the external transmission address as given above, there is no procedure to acquire the information included in the RA message, which can be received at the subnet 30 except that the RA message is passively received while being in standby after the handover.
In case MN10 may have the global IPv6 address of NAR31 of the subnet 30 before the handover, the packet transmitted from MN10 immediately after the handover can reach CN40 if encapsulation of the packet as shown in FIG. 44 is carried out by MN10. However, various prescriptions must be newly set up such as conversion of header at NAR31 or de-encapsulation of the packet at PAR21, and this requires modification of the total communication system, and it is not a very realistic solution.
In FMIP, it is possible to acquire network prefix of the subnet 30 and the link layer address of NAR31, and it is possible to estimate the link local address of NAR31 from such types of information. However, even when the link local address of NAR31 can be estimated, it is necessary to confirm whether it is used (or it is correct) or not, and this means that time is required for confirmation. Also, it is possible to estimate the link local address of NAR31 but it is entirely impossible to estimate the link local address of the default router 37 existing in the subnet 30.